Multi-bit digitally controlled current source circuits are widely used in a variety of integrated circuits, such as a current mode digital to analog converter, a current mode laser driver, and various CML circuits.
The multi-bit digitally controlled current source generally consists of a digital logic control circuit, a switch array, and a current source array. A flow chart of an n-bit digital control current source is shown in FIG. 1. An output bus of the digital logic control circuit controls turn-on or turn-off of each switch in the switch array so as to control an output current of the current source array.
FIG. 2 shows a conventional n-bit current source circuit, and both the switch array and current source array are implemented by n-channel metal oxide semiconductor (NMOS) transistor. The NMOS transistor mn receives a reference current iref and generates a bias voltage V_bias. Each current source branch in the current source array includes 2i current source units. The i-th (i=0, 1, . . . , n−1) current source branch can generate 2i·iref current under the control of V_bias. The digital control circuit outputs an n-bit control code, when Bi (i=0, 1, . . . , n−1) is logic 1, Vbi is equal to a digital power supply voltage DVDD, at this time, the i-th current source branch is turned on; and when Bi (i=0, 1, . . . , n−1) is logic 0, Vbi is equal to the ground level VSS, at this time, the i-th current source branch is turned off.
The main drawback of this topology is that the current source array's output current varies with its output voltage Vout due to the channel length modulation effect. When Vout is not equal to V_bias, the output current will deviate from a designed value, and the greater the difference between Vout and V_bias, the greater the error of the output current. If it is necessary to increase the output current accuracy, it is necessary to increase the channel length of the current mirror unit, or to use a cascode current source as shown in FIG. 3. Both of the above-mentioned two improvements increase the area of the chip and increase significantly as the bit number of n of the current source increases.